1. Field of the Invention
This invention relates to method and structure for packaging a semiconductor device, and more particularly a semiconductor device for emitting radiant energy.
2. Description of the Prior Art
In the past, thermosetting and thermoplastic materials, such as acrylics or polycarbonates, have been employed to encapsulate light emitting diode semiconductor devices. Normally, a plurality of light emitting diodes are electrically bonded to respective lead pairs on a lead frame structure. After the device bonding operation the entire lead frame structure is inserted in an injection molding apparatus and all of the devices are encapsulated by employing injection molding techniques. Thereafter, each of the lead pairs are then separated from the overall lead frame structure for use in interconnection substrates, such as, cards or boards. This batch processing reduces cost and simplifies handling during the overall encapsulating or packaging process.
Normally, when the separate devices are used on cards or boards it is necessary to connect the exposed terminal leads to other circuitry and soldering is often the preferred technique. However, the overall soldering technique requires that the in situ soldered light emitting semiconductor device be rinsed and cleaned by using a suitable solvent in order to remove the solder flux.
This procedure causes a totally unexpected problem in that while the overall packaged light emitting device was virtually indestructible during normal lifetime usage, the exposure of the packaged device to solvents in many instances completely destroyed the integrity of the injected molded outer package producing internal cracks or a "fried marble" appearance. Naturally, these defects completely destroyed the requisite light transmitting qualities of the package.
Although the phenomena is not completely understood, the injection molding of an acrylic or polycarbonate plastic material over the light emitting device apparently creates latent stresses in the device package. These latent stresses are released when exposed to certain solvents causing package damage.
One obvious suggested solution is to increase the strength of the acrylic or polycarbonate material in order to allow it to withstand the exposure to certain chemical solvents. However, this solution has its drawbacks when applied to light emitting diode devices. The addition of strengthening material to the acrylic or polycarbonate material detracts from the light or radiant energy transmitting qualities of the cured material.
In looking at the prior art, it is found that elastomeric resilient silicone resins have been employed to protect semiconductor devices per se. That is, the silicone resins provide both mechanical and device protection to the semiconductor device. However, with respect to the problem leading to the present invention, the semiconductor device was in itself undamaged by the release of these package stresses upon contact with the solvent cleaner, but the package was damaged. Moreover, it was found that if non-injection molding techniques were employed to form the polycarbonate or acrylic package, then these latent package stresses were often avoided. This latter solution of course suffers in that it loses all of the batch processing, time, and cost advantages associated with injection molding.
Further, during the analysis of the apparent mechanisms leading to package failures resulting from the release of latent internal stresses, it was found that minimal package damage was experienced even using injection molding techniques when the lead structure was constituted by non-angular straight portions. Thus, it has been theorized that the creation of these latent stresses are further aggravated by the existence of sharp structural angular lead edges.